1. Field of the Invention
The present invention relates to a developing apparatus for developing electrostatic latent images on an electrostatic latent image bearing surface such as a photosensitive or insulating material in electrophotography, electrostatic recording or the like.
2. Description of the Prior Art
In developing electrostatic latent images, a developer, i.e., a toner in charged particle form, is supplied to an electrostatic latent image bearing surface. The toner is attracted to the electrostatic latent image bearing surface in association with the electrostatic latent image pattern potential by the electrostatic attraction of the toner, thereby visualizing the electrostatic latent image. For image transfer, the toner visual image is transferred from the electrostatic latent image bearing surface to a transfer material and is then fixed by heat or pressure.
Various developing methods are known which are roughly classified into two categories: the dry developing method and the liquid (wet) developing method. The dry developing method is further classified into the two-component developing method which uses as a developer a mixture of a toner and carrier particles, and the one-component developing method which uses only a toner such as a magnetic toner. The liquid developing method uses a dispersion of a toner in a carrier solution such as a petroleum-type insulating liquid.
In all of these developing methods, a method for applying a bias voltage to the developing portion is generally adopted as a means for preventing the so-called background fog phenomenon wherein the toner becomes attached to regions other than the electrostatic latent image bearing surface region. When the bias voltage is applied, the toner does not become attached to the region of the image formation surface, that is, the electrostatic latent image bearing surface at which the surface potential is less than a predetermined threshold level, so that background fog may, thus, be prevented.
However, with this method, when the bias voltage is too low as compared with the surface potential of the electrostatic latent image bearing surface, background fog still occurs. When it is too high, the half-tone portions may become totally white or fine lines may become thinner or vanish. Therefore, the bias voltage to be applied must be carefully controlled according to the fluctuation in the surface potential of the electrostatic latent image bearing surface in order to constantly obtain images of good quality even in the case of changes in environmental factors.
Another method for preventing background fog is known which utilizes the so-called self-bias effects which are obtained when the developing electrodes for applying the bias voltage are placed under the electrically floating condition. When the developing electrodes are placed under the floating condition, the threshold level of the surface potential can be changed in accordance with the fluctuation in the surface potential of the electrostatic latent image bearing surface. In this manner, the bias voltage control need not be precise and may even be unnecessary.
It is, however, difficult in practice to effectively, stably utilize the self-bias effects.